INTRODUCTION
It is presumed that the worldâ„¢s present oil resources may be wiped out in about in about 42 years. Moreover the estimated time of the world future oil resources depend on the price demand. At the same time possibility of supply of crude oil by several times at the present price might be impossible. Therefore a new fuel should be linked to the oil resources.
Next there are problems of urban air quality standard and a global environment. The exhaust emissions from gasoline engines are becoming very clean with aid of advanced fuel formulation, intake air quantity, in-cylinder combustion control and exhaust gas after treatment technology. But the exhaust emissions of heavy-duty diesel vehicles play a massive role in environmental pollution by emitting NOx and soot (PM). The PM emissions can be reduced through engine modification such as the use of very high injection pressure etc. NOx may be reduced to some extent by retarding the injection timing or by employing exhaust gas recirculation system. However suitable catalysts are yet to be established.
DME (Dimethyl ether) is a new alternative fuel that can be used to solve the environmental pollution problems along with providing many other utilities. Several experiments have been carried out on the fuel and satisfactory results have been obtained. Lower emissions are produced and other advantages include thermal efficiency and low engine noise with DME compared to the diesel engine operating with conventional diesel fuel. The cost of DME is only 1100RMB/tonne, which is much lower than diesel on an equal energy basis. DME is an excellent and environmental friendly alternative fuel for diesel.

MANUFACTURING PROCESS OF DME
DME is manufactured presently using two different raw materials. The process is described below with their chemical formulae.
Using methanol (CH3OH) as raw material it is dehydrated to remove single water molecule from two methanol molecules. The dehydration reaction is described by the following equation.
2CH3OH CH3OH + H2O
This process is done in a micro reactor with about 200mg of catalyst. From the catalyst screening results for the conversion g-AL2O3 is favoured as the preferred one due itâ„¢s much lower cost. g-AL2O3 has the advantage of being resistant to overheating although it has the lowest activity.
The technology for direct synthesis of DME developed by NKK employs a single step reaction to synthesis DME using carbon monoxide and hydrogen gases as the raw material.

2CO + 4H2 2CH3OH (methanol) (2)
2CH3OH CH3OH (DME) + H2O (3)
H2O + CO H2 +CO2 (4)
The two molecules of methanol synthesized from CO2 and H2 in formula (2) are dehydrated in formula (3) to produce DME. The water produced in formula (3) is recycled. The hydrogen produced at the completion of the direct synthesis reaction becomes raw material for the reaction in step (2). In this way reaction cycle is formed in which the three reactions consume the by â€œproducts in each step. The reaction (1) is conducted at a pressure of 3 to7Mpa and temperature 250 to 2800c. Formula (1) is a highly exothermic reaction that produces 58.8Kcal/mole of DME. The DME direct synthesis is characterised by NKKâ„¢S proprietary highly active catalyst and the DME slurry bed reactor technology, which controls the reaction to extract the maximum performance from this catalyst.
In 1995, NKK established a small-scale bench plant capable of producing dimethyl ether at a rate of 50Kg per day. In1997 a large-scale bench plant capable of producing approximately five tons per day was constructed. The plant employs refined methane from coal layers or simulated coal gas (produced from LPG in this plant) as raw material to efficiently produce DME in a single â€œstep reaction using a slurry bed reactor.
Process components
(1) Synthesis gas production system incorporating a reformer to produce carbon monoxide and hydrogen from the coal bed methane or LPG.
(2) Reaction system for direct synthesis of dimethyl ether.
(3) Distillation system for refining the DME produced, and separating and recovering the by-product CO2.
(4) Product storage and utility equipment system.
Each unit is controlled from a central computer in the control room. The coal bed methane or LPG is burned in the auto-thermal reformer in an atmosphere of pure oxygen to produce CO and H2 in a ratio of 1:1. The CO2 produced in the reformer and the CO2 produced as by-product of direct synthesis of DME are separated and partially recycled back to the reformer for use as a raw material for gas synthesis. The large-scale bench plant is shown in photo 1.The process flow of the large-scale bench plant is shown in the figure 1.

Dimethyl ether is currently manufactured at the rate of 10,000 tonnes annually using methanol as the raw material. This is used for such applications as cosmetics and aerosol paint propellants.
CHARACTERISTICS OF DME AS A FUEL
Dimethyl ether has similar properties to LPG in that it is a gas at ambient temperature and atmospheric pressure. It becomes a colourless clear liquid under six atmospheres, at ambient temperature or at atmospheric pressure and a temperature of -25.1Ã‚Â¬Ã‚Â¬0C. Thus, DME can be transported and stored as a liquid at low temperature in a similar manner to LPG. DME is a clean fuel that contains no sulphur or nitrogen compounds, has extremely low toxicity for humans, and has no corrosive effect on metals.
Its calorific value is approximately 65% that of methane (natural gas) and approximately 40% that of methanol. Although DME has lower calorific value than LPG because of differences in the chemical structure the density of liquid DME is greater, so the total calorific value of a tank of DME is approximately 90% of that of a similar tank of LPG. When used as a replacement for diesel fuel, DME has a high cetane value, contains oxygen and has a chemical structure that forms carbon-carbon bonds, so that its combustion is not accompanied by black smoke or soot. This property has attracted considerable interest in DME as a clean fuel.
PHYSICAL AND CHEMICAL PROPERTIES OF DME AND DIESEL

From the table showing the different property values of DME and diesel the following comparisons are made.
Cetane number
Cetane number (CN) of a fuel is a measure of its propensity for auto-ignition. The CN has a strong influence on the length of time from start of fuel injection to the start of combustion in a diesel engine. The CN affects the ease of starting, the combustion generated noise, and the exhaust emissions of diesel engine. The higher CN reduces ignition delay and results in smoother combustion noise and lower combustion noise. An increase in CN produces a decrease in NOx emissions due to lower gas temperature and pressure in combustion chamber. The cetane number of dimethyl ether is higher and auto-ignition temperature is lower than that of diesel.
Boiling point
DMEâ„¢s boiling point is â€œ24.90c whereas that of diesel is 180-3600C. DME must be pressurized to be over RVP to keep it in liquid state under ambient condition. Lower value of boiling point makes the fuel to reduce aromatics and emissions of CO, HC and particulate matter. Therefore DME has the above advantages over diesel due to its low boiling point compared with diesel.
Oxygen content
Dimethyl ether has only C-H and C-O bond. It contains about 34.8% oxygen, therefore the combustion produced emissions such as CO, HC, CO2, smoke and PM are expected to be lower than diesel operation and the oxygenated fuel can tolerate more EGR ratio to reduce NOx. The ordinary diesel fuel has zero oxygen content and that of DME is 34.8% as mentioned above. Therefore using DME can reduce chances of incomplete combustion.
Latent heat of evaporation
Latent heat of evaporation of DME is much higher than that of diesel. Its value for DME is 460KJ/Kg and for diesel is 290KJ/Kg. This is beneficial to the NOx reduction due to the larger temperature drop of the mixture in the cylinder.
Low heating value
The low heat value of DME is only 64.7% of that of diesel; therefore larger amount of fuel supply is needed to deliver the same power output for the engine
Density
Density is a measure of a fuelâ„¢s mass per unit volume. Lower density fuel should reduce the level of smoke. But it will reduce power as well if the fuel injection is not set up for that lower density
Ignition temperature
The ignition temperature of diesel is 2500c and that of DME fuel is 2350C. The ignition temperature is a significant factor in developing knock in an internal combustion engine. As the ignition temperature of a fuel is high than tendency to knock is more. Therefore DME fuel is more better than diesel when knocking factor is considered.
Viscosity
Dimethyl ether has low viscosity compared to diesel. Low viscosity can cause problems like fuel leakage in the fuel supply system. But proper sealing of the fuel supply parts can avert this. About 1-2% lubricant was added in DME during a test conducted.
EXPERIMENTAL STUDY
An experimental study or test was conducted on single cylinder and multi- cylinder diesel engine using DME as fuel. The main parameters of combustion such as plunger diameter, fuel delivery advance angle, distance of nozzle tip into cylinder and swirl ratio are optimized. The combustion and emission characteristics are compared with diesel engine using diesel. A DME powered van was developed and its trial operation was carried out highway. The schematic block of the fuel supply system of DME engine is shown in figure 2.

Figure 2
Optimization of parameters of combustion system
Fuel delivery advance angle
The test results show that the optimum timing is 190CA BTDC at rated operating conditions and 150CA BTDC at low speed respectively due to the shorter ignition delay of DME
Air swirl ratio
The optimum air swirl ratio for the highest thermal efficiency of the engine is ranging from 1.4 to 1.8, which is lower than that of diesel operation (2.3) due to easy evaporation of DME and mixing with air.

Figure 3
Diameter of plunger
Three diameters of plunger were tested, 8.5mm, 9.0mm and 9.5mm respectively. Thermal efficiency increased with the enlargement of plunger diameter. This is because the heating value and density of DME is only 64.7% and 80% of that of diesel respectively. The larger diameter plunger will shorten the injection duration to improve the engine performances of DME engine.

Figure 4
Distance of nozzle tip into cylinder
The optimum distance for DME operation is 5mm, which is larger than that of diesel operation (3mm). This is because DME has excellent evaporation characteristics and also the spray angle becomes much larger soon after being injected into the cylinder.
Injection pressure
The test results demonstrate that the optimum pressure is 15Mpa for DME, which is lower than that of diesel engines due to its physical properties. At 15MPa the thermal efficiency is maximum as observed from the figure 5

Figure 5
Combustion characteristics
The figure 6 shows the indicator diagrams and rates of pressure rise of DME and diesel engine. It can be seen that DME engine has lower maximum cylinder pressure and much lower rate of pressure rise compared with that of diesel engine. Therefore DME engine has the advantages of low mechanical load and lower combustion noise.
Comparison of pressure and its rise operating on diesel and DME (diesel-left; DME-right)
Figure 6
Operational results of DME powered vehicle
The noise of the vehicle is equivalent to that of gasoline powered vehicle under frequently operating speed (90Km) and smoke free combustion is realized. The maximum speed (115-120Km/hr) and accelerating performance are equivalent to that of diesel operation. No apparent wear is observed by adding some lubricant in DME. The DME fuelled engine produced a lager amount of mechanical work and a smaller amount of wasted internal energy in the exhaust.
DME UTILISATION TECHNOLOGY
Use as an LPG alternative
The prospect for dimethyl ether as an alternative fuel to LPG in consumer applications stimulated experiments to investigate DME combustion in existing mass-produced combustion equipment in order to gain an understanding of its combustion characteristics. DME has a WI (wobbe index, which is the higher calorific value/ (specific gravity) .5 and is used as a basic measure of heat input for gas appliances) of 51.91MJ/Nm3 and an MCP (an index of the rate of gas combustion) of 48 to 50.DME is equivalent to a gas classification of 12C, a type not currently used. Combustion of DME with the municipal gas 13A was relatively satisfactory, but the flame tended to be shorter than the case when 13A is burned. Changing the opening of the variable air adjustment damper from 8/10 to 5/10 reduced the primary air supply and resulted in good combustion. JIS combustion testing to determine the suitability of the gas combustion equipment for DME resulted in passes in all items on the test schedule, from test in no-wind conditions to test with large pots and certification was granted. The details are shown in table 3.
Table 3 DME combustion test results

Use as a diesel fuel alternative
DME has the same or higher cetane value as diesel fuel and is therefore ideal for use in diesel engines. However modifications to the fuel supply system are required because both the boiling point and viscosity of DME are less than that of diesel fuel. In 1997 a diesel engine test bench was made to investigate both the characteristics of DME when used in diesel engines and the fuel supply system required for this application. In 1998 a light truck was operated on DME, which was the world first. A description of the modifications to the diesel engines for using DME and the associated combustion characteristics are as follows. The use of DME, with its low boiling point and low viscosity, does not require modifications to the diesel engine itself. Nevertheless, in contrast to diesel fuel, DME must be supplied to the fuel injection pump under pressure. In initial experiments, gas pressure from a nitrogen bottle was used for pressurization, but this installation resulted in supply equipment of considerable size, and nitrogen gas dissolved into the liquid DME. A small fuel supply pump was developed to overcome these problems. The original diesel fuel tank was used to contain DME.
The combustion characteristics of the DME engine include a thermal efficiency that is similar to that of diesel, but without soot emission and with greatly reduced NOx emissions and noise levels. An oxidation catalyst is sufficient for purifying the exhaust gas because the engine does not emit black smoke and the CO and HC emissions are greatly reduced. The modifications to a diesel engine to permit the use of DME are minimal, consisting only of changes to the fuel supply system. The combustion characteristics of dimethyl ether and diesel fuel are shown in figure 7.

Figure 7 DME and diesel fuel performance comparison (at 2000rpm)
APPLICATIONS
DME is a fuel having versatile utilization. DME is already used as an aerosol propellant. It is used as a propellant for cosmetic and coating material and worldwide requirement is about 1,50,000tonnes/year. Presently, Japan has a requirement of 10,000tonnes/year. This utilization is the substitution of Freon, which destroys the ozone layer.
It is expected that, in the future, DME will also be used in such applications as fuel for power generation, diesel vehicles and consumer applications, as a raw material for chemical processes and as a cooling medium. In future DME can be produced from biomass.
DISADVANTAGES
The weak points of DME as a fuel are poor lubrication and viscosity. Another weak point of DME as a fuel is the relatively low heating value. Also there is no infrastructure of fueling system of DME. Dimethyl ether has a high vapour pressure that requires a special handling such as a pressurized fuel container to avoid the fire hazard. Because of low boiling point temperature, DME is likely to be subjected to a supercritical point in the CI engine injector. Then, when fuel is suddenly issued out of injector, cavitation is expected to accompany the fuel flow that would damage the nozzle hole.
By using dimethyl ether-diesel blended fuel the viscosity and low heating value can be increased. When DME is mixed with diesel fuel (60:40 v/v), the fuel leakage can be decreased by about 7 percent that is half of the neat DME operation.
CONCLUSION
Based on experimental studies and evaluation dimethyl ether is an excellent and environmental friendly alternative fuel for diesel engines. The toxicity of DME is lower than that of methanol. There is no green house effect and problem of ozone layer depletion. The relatively high cetane number and low soot emission offset the disadvantages of the fuel for the use in CI engine. The cost of dimethyl ether is much lower than the diesel fuel on an equal energy basis. Also the fuel has a versatile utility and can be derived from abundant natural resources such as natural gas coal and vegetation (via methanol). Thus dimethyl ether has all the qualities to satisfy the requirements of an alternative energy source, especially for automobiles. It is hoped that the use of DME vehicles will become widespread in the near future.
REFERENCES
Suichi Kajitani and Zhili Chen, Fundamental research on next generation fuel (dimethyl ether) engines, Journal of Scientific and Industrial research, vol.62, No.2, pp133-144, 2003.
Zhou Longbao, Wang Hewu, and Wang Ying, Experimental study on performances and combustion characteristics of DME powered vehicle, SAE paper 950064
Yotoro Ohno, Norio Inoue, Takashi Ogawa, Masami Ono, Tsutomo Shikada and Hiromasa Hayashi, Slurry phase synthesis and utilization of dimethyl ether, SAE Paper 950064
G. Bercic, Catalytic dehydration of methanol to dimethyl ether, kinetic investigation and reactor simulation, Ind. Eng. Chem.res, vol.32, pp 2478-2484,1993
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ABSTRACT

It is presumed that the worldâ„¢s present oil resources may be wiped out in about in about 42 years. Moreover the estimated time of the world future oil resources depend on the price demand. At the same time possibility of supply of crude oil by several times at the present price might be impossible. Therefore a new fuel should be linked to the oil resources.

DME (Dimethyl ether) is a new alternative fuel that can be used to solve the environmental pollution problems along with providing many other utilities. Several experiments have been carried out on the fuel and satisfactory results have been obtained. Lower emissions are produced and other advantages include thermal efficiency and low engine noise with DME compared to the diesel engine operating with conventional diesel fuel. The cost of DME is only 1100RMB/tonne, which is much lower than diesel on an equal energy basis. DME is an excellent and environmental friendly alternative fuel for diesel.

=Need for clean and pollution free environment.
=World present oil resources may be wiped out in 42 years
=Diesel engine operated on DME fuel has low combustion noise and low emission.
=Fuel is economical and has many utilities.

MANUFACTURING PROCESS OF DME

1.Catalytic dehydration of methanol to DME

2CH3OH CH3OCH3+H2O

2.Direct synthesis of DME

3CO +3H2 CH3OCH3+CO2

CHARACTERISTICS OF DME AS A FUEL

=Gas at ambient temperature and atmospheric pressure.
=Colourless clear liquid under six atmospheres.
=Can be transported and stored as a liquid at low temperatures.
=No sulphur or nitrogen compounds and low toxicity.
=No corrosive effect on metals.
=Oxygenated fuel.
=Combustion is not accompanied by black smoke or soot.